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RH15 %
WindNE 18 mph
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Hawaiian Islands Synoptic Discussion and Guidance

421
FXHW60 PHFO 231403
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
403 AM HST Tue Oct 23 2018

.SYNOPSIS...
A surface ridge north of the islands will maintain locally breezy
trade winds into mid-week. A rather tranquil trade wind weather
pattern is also expected to persist across most of the state this
week. However, enhanced moisture just upstream of the eastern end
of the state is spreading toward the Big Island. As a result,
showers are expected to increase along the windward sides of the
Big Island and Maui starting tonight. The trade winds will weaken
slightly on Thursday and Friday, followed by stronger trades on
Saturday. The flow will begin to turn southeasterly late Sunday
and Monday, with the potential for much wetter conditions
developing across much of the island chain early next week.

&&

.DISCUSSION...
A 1020 mb surface high is centered near 27N 170W, or about 480
miles east of Midway Atoll. A surface ridge extends from this
feature through a point about 450 miles north of Honolulu.
Elsewhere, a weak elongated surface trough is evident around 370
miles southeast of Hilo. The Hawaiian Islands remain sandwiched
between these systems, which is producing a relatively tight
pressure gradient across the region early this morning. As a
result, locally breezy trade winds persist across the state.
Aloft, weak middle to upper tropospheric troughing remains just
east of the Big Island. The low-level trade wind inversion has
become established around 6 or 7 thousand feet according to the
morning balloon soundings at Hilo and Lihue. But it appears to be
relatively weak.

A lingering area of slightly enhanced moisture is evident just
northeast of Maui. There is also a large area of enhanced
moisture about 50 or 60 miles east and southeast of the Big
Island. Relatively dry air is over the western end of the state.
Low clouds and showers continue to move down across the windward
waters adjacent to Maui County this morning. These clouds and
showers are affecting windward sections of Maui and Molokai.
Additional low clouds and showers are moving into the the Puna and
Hilo Districts of the Big Island. A few low clouds with embedded
isolated showers have also been moving into the windward sides of
Oahu, while Kauai appears to be rather dry at the moment.

The latest output from the forecast models continues to show the
surface ridge will remain north of the state into mid-week before
it begins to weaken. At the same time, the weak surface trough
southeast of the Big Island will move slowly westward. This will
likely keep locally breezy trade winds in place through Wednesday.
The models also appear to show a middle tropospheric ridge nosing
in from the west over the islands during the next couple of days.
This may lead to slightly more stable atmospheric conditions,
with the trade wind inversion continuing to become a little
better defined. However, the area of enhanced moisture just
upstream of the Big Island, including along the northern fringes
of the weak surface trough described above, will likely spread
across parts of the state later today and tonight. There remains
some uncertainty in how much of this moisture will reach the
smaller islands. The latest forecast indicates that low clouds
and showers will increase over the windward Big Island, and
possibly Maui, from tonight into Wednesday.

As the current ridge north of the region weakens later this week,
the trades will become somewhat lighter on Thursday and Friday.
At the same time, a new subtropical ridge is expected to develop
far northwest and north of the state behind a weakening front that
will push down from the north-northwest. The models do not appear
to bring this frontal boundary into the islands, but low-level
moisture convergence associated with the frontal remnants may get
close to Kauai this weekend. The higher pressure to the north of
the area will cause the trades to strengthen on Saturday.

A significant change in the weather pattern appears to be likely
starting later this weekend. The surface high north of the islands
will move rapidly eastward on Sunday. At the same time, the
forecast models indicate a large low pressure area will likely
develop far northwest of the state. This will result in the
background flow across the region becoming southeasterly from
late Sunday into Monday. This scenario would allow the deep
tropical moisture located south and southeast of the state to
eventually reach the islands. It is still too early to know
exactly how this potentially wetter weather pattern may affect the
islands early next week. For now, we believe the low-level
convergence will initially increase near Kauai Therefore,
rainfall chances are highest there on Monday. The wet weather
then appears to spread across most of the rest of the state
Monday night and Tuesday.

&&

.AVIATION...
A high pressure ridge north of the islands will continue to
provide moderate trade winds to the area for the next couple of
days. Passing low clouds and showers carried by the winds will
affect mainly windward and mountain areas, bringing brief MVFR
ceiling. Otherwise VFR conditions are expected.

No AIRMET currently is in effect. AIRMET TANGO may be posted
later today or Wednesday as trade winds may strengthen enough for
low-level mechanical turbulence to develop over mountains and
south through west sections of all islands

&&

.MARINE...
A Small Craft Advisory remains in effect for the windier areas
near Maui and the Big Island through Wednesday night. The pressure
gradient over the islands should weaken Wednesday night as the
weak trough passing south of the region weakens.

The High Surf Advisory for south facing shores has been extended
through Wednesday afternoon. South swell is expected to decline
today and tonight, with marginal advisory level surf expected
today. However, a new long-period south-southwest swell is
expected to arrive Wednesday, which could keep surf at advisory
levels along south facing shores through Wednesday afternoon. A
reinforcing south swell arriving Friday will likely keep surf
elevated along south facing shores as we head into the weekend.

Surf along remaining shores is expected to remain below advisory
levels through the end of the work week. Rough surf along east
facing shores will increase slightly during the next few days due
to the locally strong trade winds. A moderate north-northwest
swell arriving today is expected to peak tonight or Wednesday.
This swell will slowly decline Wednesday night. A reinforcing
north-northwest swell arriving Thursday will maintain moderate
surf along most north and west facing shores through early
Saturday. A new north-northwest swell arriving late Saturday will
likely cause surf to build along most north and west facing shores
from Saturday night through Sunday.

See the latest Collaborative Nearshore Swell and Wind Forecast
for Oahu (SRDHFO) for additional details on surf and swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Wednesday evening for south
facing shores of all Hawaiian Islands.

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$


DISCUSSION...Houston
AVIATION...Hui
MARINE...Powell

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office